A monolithic piezoelectric filter is generally defined as a filter whose operations are based on the use of deposited rectangular or square electrode pairs acting as thickness or shear mode resonators or poles separated by nonelectrode regions which act as acoustic coupling elements. The entire filter is on a single quartz blank. The quartz crystal blank or plate forms the most frequency stable type of piezoelectric resonator.
One of the major problems encountered with piezoelectric monolithic filters is the first highest unwanted offband spurious response, clustered around the high-side of the fundamental response or pass-band of the filter. An unbalanced vibration of the quartz crystal plate will normally create this mode of resonance in non-ideal crystal blanks.
The exact level and location of this undesirable unbalanced vibrating plate spurious response (spur), is difficult to predict in advance, before the physical piezoelectric device is actually built because of variations in processing and other limitations in manufacturing capabilities. One of the only ways to reduce this unwanted spurious response is by experimentation.
Hence, there exists a need for an improved arrangement for fabricating piezoelectric monolithic filters, such as quartz crystal blanks, in which the first unwanted offband spurious response can be minimized. Therefore, a structure which helps minimize this first highest off-band spur, would be considered an improvement in the art.